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An analytical model for the acoustic radiation from finite cylindrical ducts with infinite flanges is presented in this article. This model will be used for the design of low-noise gradient coils for MRI scanners. The expression of the sound field inside the duct satisfies the boundary conditions at the wall and at the open ends. The wave reflection phenomenon at the open ends of finite ducts is described by the general radiation impedance, which links the model pressure and axial velocity at the interface using an infinite matrix expression. The results obtained from the analytical model are compared with those calculated using the commercial code based on the Boundary Element Method (BEM) and good agreement is achieved. The results reveal that the noise peaks are closely related to the resonant (cut-off) frequencies of the duct. This discovery will be useful for MRI scanner designers to avoid inherently noisy gradient coil designs. The analytical model also displays significant advantages over the BEM model with its much better numerical efficiency and direct relationship between the design parameters and the sound field inside the MRI scanner.

Abstract-This paper investigates the application of frequencyselective surface (FSS) in reflectarray antennas for the purpose of reducing radar cross section (RCS) level. Different from previous reports, the presented band-stop FSS structure is also characterized by the suppression of surface waves, which makes a contribution to better radiation performance. Two 14×14 reflectarray antennas backed on the FSS ground and a solid ground are designed and fabricated. Simulated and measured results show that the FSS ground can improve the 'inband' gain by 1.1 dB, decrease the sidelobe level by 6.4 dB, and reduce the 'out-of-band' RCS effectively when compared with the antenna with a solid ground plane of the same size.